We propose to apply the fast di-I tract-tracing method, which normally requires 3-4 months to penetrate 1 cm, to extremely rapid (1-2 hours) human brain autopsy material immediately after the brain has been removed, delaying fixation until some 24 hours later. Our preliminary studies with delayed-fixation human brain samples show 5- to 10-fold more lengthy tracer penetration distances in only 24 hours than the distances achieved in previously published studies with fixed material and three to four months penetration time. Specific aim 1 seeks to confirm and extend these data using primary visual cortex as a model system. Specific Aim 2 tests the maximum projection distance obtainable with the method using a motor cortex model. Specific Aim 3 seeks to optimize the method. Despite limitations, the delayed- fixation di-I approach should provide a useful tool for defining human brain cytoarchitecture and may be helpful in studies of specific vulnerabilities of pathways in neurologic disease.